Bmw 525tds Overheating Problem

I have a 1994 BMW 525tds with an M51 engine withn a serious overheating problem. it will run fine in traffic so it is not a problem with the fan but now and then when driving at 40 or 50 mph the temperature will quickly rise. It has also had the thromostat replaceed and the head gasget is good. there is also a good pressure in the pipes. does ne1 have ne idea wat could be wrong??????? plz help im all outa ideas!!! THANKX